Add :save and :pipe commands to viewer

* :save takes a path and saves the current message part to that location
* :pipe is the same as pipe on the account page, but uses the current
  message part rather than the whole email (ie :pipe gzip -d)
* Refactored account:pipe and extracted common pipe code to
  commands.util.QuickTerm
* Added helper command aerc.PushError

commands/util.go

@@ -0,0 +1,56 @@
+package commands
+
+import (
+	"io"
+	"os/exec"
+	"time"
+
+	"git.sr.ht/~sircmpwn/aerc/widgets"
+	"github.com/gdamore/tcell"
+)
+
+// QuickTerm is an ephemeral terminal for running a single command and quiting.
+func QuickTerm(aerc *widgets.Aerc, args []string, stdin io.Reader) (*widgets.Terminal, error) {
+	cmd := exec.Command(args[0], args[1:]...)
+	pipe, err := cmd.StdinPipe()
+	if err != nil {
+		return nil, err
+	}
+
+	term, err := widgets.NewTerminal(cmd)
+	if err != nil {
+		return nil, err
+	}
+
+	term.OnClose = func(err error) {
+		if err != nil {
+			aerc.PushError(" " + err.Error())
+			// remove the tab on error, otherwise it gets stuck
+			aerc.RemoveTab(term)
+		} else {
+			aerc.PushStatus("Process complete, press any key to close.",
+				10*time.Second)
+			term.OnEvent = func(event tcell.Event) bool {
+				aerc.RemoveTab(term)
+				return true
+			}
+		}
+	}
+
+	term.OnStart = func() {
+		status := make(chan error, 1)
+
+		go func() {
+			_, err := io.Copy(pipe, stdin)
+			defer pipe.Close()
+			status <- err
+		}()
+
+		err := <-status
+		if err != nil {
+			aerc.PushError(" " + err.Error())
+		}
+	}
+
+	return term, nil
+}